(a) Technical Field
The present invention relates to a protective film for a polarizer with superior optical and mechanical properties, a polarizing plate including the same and a display device including the same.
(b) Background Art
Recently, interests in a polarizing plate, an essential component of a liquid crystal display (LCD), are increasing as the demand on the liquid crystal display increases rapidly.
The polarizing plate, which polarizes incident natural light oscillating in various directions into light oscillating in one direction only, is an essential component for providing transmitted light and changing the color tone of the transmitted light.
The polarizing plate has a structure in which a protective film is stacked on one or both sides of a polarizer. As the polarizer, polyvinyl alcohol (PVA) is commonly used. In the past, triacetyl cellulose (TAC) was commonly used as the protective film.
Meanwhile, as the functions and applications of the liquid crystal display (LCD) become more diversified, normal operation under harsh conditions is required. However, triacetyl cellulose (TAC) does not satisfy this requirement because it is vulnerable to moisture and has weak durability.
Recently, there have been many attempts to replace the triacetyl cellulose (TAC) with polyethylene terephthalate (PET) as in Japanese Patent Application No. 2011-532061 and Japanese Patent Publication No. 2010-118509. It is because polyethylene terephthalate (PET) can satisfy the above requirement because it has superior mechanical property, chemical resistance and moisture barrier property.
However, because polyethylene terephthalate (PET) is highly birefringent, it leads to distorted polarization between the polarizer and the liquid crystal and, accordingly, significantly reduces visibility. A typical example is the rainbow stains appearing on the surface of the protective film.
Because the rainbow stains become easily visible due to the recent trend of high brightness and high color purity of the liquid crystal display (LCD), the rainbow stains are a big obstacle to use of the polyethylene terephthalate (PET) for the protective film.